Retroreflective articles having composite cube-corners and methods of making

ABSTRACT

Retroreflective articles and methods of making the same, wherein the retroreflective articles ( 10 ) include composite cube-corner elements ( 12 ) having a first light transmissive polymeric layer ( 30 ), a second light transmissive polymeric layer ( 32 ), and an interface therebetween, wherein the first light transmissive polymeric layer has a first index of refraction, the second light transmissive polymeric layer has a second index of refraction, and the first and second indices of refraction have an absolute difference of at least 0.0002.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a national stage filing under 35 U.S.C. 371 ofPCT/US2012/039024, filed May 23, 2012, which claims priority to U.S.Provisional Patent Application No. 61/491554, filed May 31, 2011, thedisclosure of which are incorporated by reference in their entiretyherein.

BACKGROUND

Retroreflective materials have the ability to redirect light incident onthe material back toward the originating light source. This property hasled to the widespread use of retroreflective sheeting for a variety oftraffic and personal safety uses. Retroreflective sheeting is commonlyemployed in a variety of articles (e.g., road signs, barricades, licenseplates, pavement markers, and pavement marking tape, as well asretroreflective tapes for vehicles and clothing).

There are generally two types of retroreflective sheeting: beadedsheeting and cube-corner sheeting. Beaded sheeting typically employs amultitude of glass or ceramic microspheres to retroreflect incidentlight. Cube-corner sheeting, on the other hand, typically employs amultitude of rigid, interconnected, cube-corner elements to retroreflectincident light. Cube-corner retroreflective sheeting, sometimes referredto as “prismatic” sheeting, typically comprises a thin transparent layerhaving a substantially planar first surface and a second structuredsurface comprising a plurality of geometric structures. In truncatedcube-corner sheeting some or all geometric structures include threereflective faces configured as a cube-corner element.

The base edges of adjacent cube-corner elements of truncated cube-cornerarrays are typically coplanar. Other cube-corner element structures,described as “full cubes” or “preferred geometry” typically comprise atleast two non-dihedral edges that are not coplanar. Such structurestypically exhibit a higher total light return in comparison to truncatedcube-corner elements. Cube-corner sheeting having “preferred geometry”cube-corner elements may be manufactured by a laminae process (see,e.g., U.S. Pat. No. 7,156,527 (Smith)).

Cube-corner retroreflective sheeting is commonly produced by firstmanufacturing a molding tool that has a structured surface, wherein thestructured surface corresponds either to the desired cube-corner elementgeometry in the finished sheeting or to a negative (inverted) copythereof, depending upon whether the finished sheeting is to havecube-corner pyramids or cube-corner cavities (or both). The molding toolis then replicated using any suitable technique such as conventionalnickel electroforming to produce tooling for forming cube-cornerretroreflective sheeting by processes such as embossing, extruding, orcast-and-curing.

Alternative retroreflective articles and alternative methods for makingretroreflective articles are desirable.

SUMMARY

In one aspect, the present disclosure describes a retroreflectivearticle having a light transmissive support layer, the lighttransmissive support layer having generally opposed first and secondmajor surfaces, and an array of composite cube-corner elements securedto the first major surface of the light transmissive support layer,wherein each composite cube-corner element has an apex and a baseopposite the apex, and wherein each composite cube-corner element has afirst light transmissive polymeric layer, a second light transmissivepolymeric layer, and an interface therebetween. The first lighttransmissive polymeric layer includes the apex, and the second lighttransmissive polymeric layer includes at least a portion of the base,wherein the first light transmissive polymeric layer has a first indexof refraction, wherein the second light transmissive polymeric layer hasa second index of refraction, and wherein the first and second indicesof refraction have an absolute difference of at least 0.0002.

In another aspect, the present disclosure describes a method of making aretroreflective article, the method including providing a molding toolhaving a microstructured surface including a plurality of cavities,partially filling the plurality of cavities with a first radiationcurable resin, wherein the at least a portion of the plurality ofcavities includes a cube-corner geometry, exposing the first radiationcurable resin to a first irradiation to pre-cure the first radiationcurable resin and provide pre-cured partial cube-corner structures,contacting a second radiation curable resin onto the pre-cured partialcube-corner structures, exposing the pre-cured partial cube-cornerstructures and the second radiation curable resin to a secondirradiation to provide composite cube-corners on the surface of themolding tool, and separating the composite cube-corners from the surfaceof the molding tool.

In another aspect, the present disclosure describes retroreflectivearticle having a body layer having generally opposed first and secondmajor surfaces, and an array of composite cube-corner elements on thefirst major surface of the body layer, wherein each compositecube-corner element comprises an apex and a base opposite the apex,wherein each composite cube-corner element comprises a first lighttransmissive polymeric layer, a second light transmissive polymericlayer, and an interface therebetween, wherein the first lighttransmissive polymeric layer comprises the apex, wherein the secondlight transmissive polymeric layer is contiguous with the body layer,wherein the first light transmissive polymeric layer has a first indexof refraction, wherein the second light transmissive polymeric layer hasa second index of refraction, and wherein the first and second indicesof refraction have an absolute difference of at least 0.0002.

In another aspect, the present disclosure describes a method of making aretroreflective article, the method including providing a molding toolhaving a microstructured surface including a plurality of cube-cornercavities, applying a first radiation curable resin to a portion of thecube-corner cavities in a desired pattern, partially filling a portionof the cube-corner cavities and forming partially filled cube cornercavities and unfilled cube-corner cavities, contacting the partiallyfilled cube corner cavities and unfilled cube-corner cavities with asecond radiation curable resin, wherein the second radiation curableresin is different from the first radiation curable resin, forming acomposite, exposing the composite to an irradiation source to providecomposite cube-corners and monolithic cube-corners on the surface of themolding tool, and separating the composite cube-corners and monolithiccube-corners from the surface of the molding tool.

In another aspect, the present disclosure describes a method of making aretroreflective article, the method including providing a molding toolhaving a microstructured surface including a plurality of cube-cornercavities, applying a first radiation curable resin to a portion of thecube-corner cavities in a desired pattern, partially filling a portionof the cavities and forming partially filled cube corner cavities andunfilled cube-corner cavities, exposing the composite to a firstirradiation to provide pre-cured partial cube-corner structures,contacting the pre-cured partial cube-corner structures and unfilledcube-corner cavities with a second radiation curable resin, forming acomposite, exposing the composite to a second irradiation to providecomposite cube-corners and monolithic cube-corners on the surface of themolding tool, and separating the composite cube-corners and monolithiccube-corners from the surface of the molding tool.

In another aspect, the present disclosure describes a retroreflectivearticle comprising a light transmissive support layer having generallyopposed first and second major surfaces, an array of compositecube-corner elements and monolithic cube-corner elements secured to thefirst major surface of the light transmissive support layer, eachcube-corner element comprising an apex and a base opposite the apex,wherein each composite cube-corner element comprises a first lighttransmissive polymeric layer comprising the apex, the first lighttransmissive polymeric layer having a first index of refraction, and asecond light transmissive polymeric comprising at least a portion of thebase, the second light transmissive polymeric layer having a secondindex of refraction, and wherein the first and second indices ofrefraction have an absolute difference of at least 0.0002.

“Cube-corner elements” refers to any arrangement, constituent of opticalshape, or structure capable of retroreflecting incident light. Theretroreflective structure includes cube-corner type trigonal pyramids,cube-corner type cavities, cube-corner type trigonal pyramids withreflective layers, cube-corner type cavities with reflective layers, andfull cubes.

“Light transmissive” refers to a material that transmits at least 70% ofthe intensity of an incident light of a given wavelength (in someembodiments at least 80% or even at least 90% of the intensity of anincident light of a given wavelength).

“Microstructure”, used herein in the context of an article having asurface bearing microstructure, refers to the configuration of a surfacewhich depicts or characterizes the predetermined desired utilitarianpurpose or function of the article. Discontinuities (e.g., projectionsand indentations in the surface) will deviate in profile from theaverage profile or center line drawn through the microstructure suchthat the sum of the areas embraced by the surface profile above the lineis equal to the sum of those areas below the line, the line beingessentially parallel to the nominal surface (bearing the microstructure)of the article. The heights of the deviations are ±0.005 micrometer to±750 micrometers through a representative characteristic length of thesurface (e.g., 1 centimeter to 30 centimeters). The average profile, orcenter line, can be plano, concave, convex, aspheric, or combinationsthereof. Articles where the deviations are of low order (e.g., from±0.005 micrometer to ±0.1 micrometer or, preferably, from ±0.005micrometer to ±0.05 micrometers) and the deviations are of infrequent orminimal occurrence (i.e., the surface is free of any significantdiscontinuities), are those where the microstructure-bearing surface isan essentially “flat” or “perfectly smooth” surface, such articles beinguseful, for example, as precision optical elements or elements with aprecision optical interface (e.g., ophthalmic lenses). Articles wherethe deviations are of the low order and of frequent occurrence are thosebearing utilitarian discontinuities, as in the case of articles havinganti-reflective microstructure. Articles where the deviations are ofhigh order (e.g., from ±0.1 micrometer to ±750 micrometer) andattributable to microstructure comprising a plurality of utilitariandiscontinuities which are the same or different and spaced apart orcontiguous in a random or ordered manner, are articles (e.g.,retroreflective cube-corner sheeting, linear Fresnel lenses, and videodiscs). The microstructure-bearing surface can contain utilitariandiscontinuities of both the low and high orders. Themicrostructure-bearing surface may contain extraneous or non-utilitariandiscontinuities so long as the amounts or types thereof do notsignificantly interfere with or adversely affect the predetermineddesired utilities of the articles. In some embodiments, microstructuralelements include at least one of cones, diffraction gratings,lenticulars, segments of a sphere, pyramids, cylinders, fresnels, orprisms. It may be necessary or desirable to select a particularoligomeric composition whose shrinkage upon curing does not result inthe interfering extraneous discontinuities (e.g., a composition whichshrinks only 2% to 6%). The profiles and the dimensions and spacing ofthe discontinuities are those discernible by an electron microscope at1000× to 100,000×, or an optical microscope at 10× to 1000×.

“Optically variable mark” refers to a retroreflective mark exhibiting avarying appearance depending on, for example, the angle at which themark is viewed, the type of light that is used to view theretroreflective mark (e.g., reflective light versus transmissive light,visible versus non-visible).

“Pre-cured” refers to a portion of the radiation curable, cross-linkableand/or reactable components in a radiation curable resin that have beenexposed to an amount of radiation sufficient to partially cure,cross-link and/or react the radiation curable resin. In alternateembodiments of the present disclosure, the degree of reacted componentscan vary widely. A pre-cured radiation curable resin can be furthercured to improve polymer properties (e.g., hardness and refractiveindex).

“Sheeting” refers to a thin piece of polymeric (e.g., synthetic)material. The sheeting may be of any width and length, such dimensiononly being limited by the equipment (e.g., width of the molding tool, orwidth of the slot die orifice) from which the sheeting was made.

“Visible” refers to being apparent and identifiable (i.e., to ascertaindefinitive characteristics of) to the unaided human eye of normal (i.e.,20/20) vision. By “unaided”, it is meant without the use of a microscopeor magnifying glass.

Exemplary uses of retroreflective articles described herein includetraffic control signs, vehicle license plates, and conspicuity films.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a diagram illustrating the nature and geometry ofretroreflection;

FIG. 1B is a cross-sectional view of Prior Art retroreflective sheeting;

FIG. 2 is an exemplary embodiment of an article disclosed herein havingan array of composite cube-corner elements;

FIG. 3 is a cross-sectional view of the array of composite cube-cornerelements in FIG. 2;

FIG. 4 is a schematic representation of a side view of a retroreflectivearticle having composite cube-corner elements;

FIGS. 5A-5F are schematic representations of a cube-corner recess in amolding tool in the progressive deposition and subsequent partial curingfrom irradiation which occur during the production of a compositecube-corner;

FIG. 6 is a schematic representation of an exemplary processconfiguration for practice of a method of making a retroreflectivearticle having a two-dimensional array of composite cured cube-cornerelements;

FIG. 7 is a graphical representation of Percent Light Retention (%R_(T)) versus Observation Angle for exemplary retroreflective articlesdescribed herein;

FIG. 8 is a graphical representation of Percent Light Retention slope (%R_(T) slope) versus Observation Angle for exemplary retroreflectivearticles described herein; and

FIG. 9 is a schematic representation of a side view of a retroreflectivearticle having a combination of composite cube-corner elements andmonolithic cube-corner elements.

Like reference numbers in the various figures indicate like elements.However, it will be understood that the use of a number to refer to acomponent in a given figure is not intended to limit the component inanother figure labeled with the same number. Some elements may bepresent in identical or equivalent multiples; in such cases only one ormore representative elements may be designated by a reference number butit will be understood that such reference numbers apply to all suchidentical elements. Unless otherwise indicated, all figures and drawingsin this document are not to scale and are chosen for the purpose ofillustrating different embodiments of the description. In particular thedimensions of the various components are depicted in illustrative termsonly, and no relationship between the dimensions of the variouscomponents should be inferred from the drawings, unless so indicated.Although terms such as “top”, bottom”, “upper”, lower”, “under”, “over”,“front”, “back”, “outward”, “inward”, “up” and “down”, and “first” and“second” may be used in this disclosure, it should be understood thatthose terms are used in their relative sense only unless otherwisenoted. In particular, in some embodiments certain components may bepresent in interchangeable and/or identical multiples (e.g., pairs). Forthese components, the designation of “first” and “second” may apply tothe order of use, as noted herein (with it being irrelevant as to whichone of the components is selected to be used first).

DETAILED DESCRIPTION

In FIG. 1A, retroreflective surface 14 is shown with a ray or pencil ofrays of light 16 coming from a distant source such as a vehicleheadlight (not shown) and impinging on surface 14 at entrance angle, β,(the angle between incident ray 16 and normal 18 to surface 14). Ifsurface 14 was an ordinary mirror that produced specular reflection, theemergent or reflected rays would leave surface 14 at the same angle buton the opposite side of the normal (not shown). If surface 14 was adiffuse reflector, emergent or reflected rays would go offindiscriminately in all directions (not shown) and only a small fractionwould return to the source. However, with retroreflection there is adirectional reflection by the surface such that a cone of brilliantlight is returned toward the source, the axis of the cone beingsubstantially the same as the axis of incident ray 16. By “cone ofbrilliant light” it is meant that the intensity of light within the coneis greater that would be the case where diffuse reflection occurs. Thismay hold true only where entrance angle, β, of the light does not exceeda certain value depending upon the characteristics of surface 14.

An observer's eye is seldom on the axis of incident light ray 16. Thusin the case of an automobile approaching a highway sign, there will bean angle between any given ray of incident light approaching the signfrom each headlight and the reflective rays reaching the driver's eyes.Hence if the retroreflective surface is perfect in directional action,with incident light being returned only toward its source, it would havelittle or no utility as a sign. There should be an expansion orspreading out of retroreflected light rays in order that persons near,but off, the axis of the incident light may take advantage of theretroreflective characteristic of the reflector or sign, but thisexpansion should not be excessive or the retroreflective brightness ofthe sign will suffer through diffusion of reflected light outside theuseful range. The deviation of particular ray 20 which is visible to anoccupant of the car whose headlight emitted pencil of light rays 16 isillustrated in FIG. 1A. The acute angle between incident ray 16 andemergent ray 20 is designated as observation or divergence angle, α.

At great distances (e.g., several hundred meters or more), most vehiclespresent a similar observation angle geometry. However, at closerdistances the configuration of a vehicle (i.e., the relative location ofand distances between each headlight and the driver's eyes with regardto the sign) becomes more significant. For instance, at a distance ofabout 30 meters from a sign, for the driver of a typical automobile, theobservation angle for light from a headlight to the driver's eyes isabout 1° whereas for the driver of a large truck the observation anglemay be substantially larger (e.g., about 3°). In order for the sign tobe effective for the driver of the truck, the observation angle ofretroreflected light from the truck's headlights must be reflected at agreater observation angle (i.e., the cone of brilliant light must bebroader) than is necessary to benefit the driver of an automobile. It istherefore desirable to be able to obtain retroreflective sheeting havinga broader cone of retroreflected light, as is provided by theretroreflective sheeting described herein, having compositecube-corners.

FIG. 1B illustrate light ray 160 being retroreflected by prior artcube-corner retroreflective sheeting 100. Prior art cube-cornerretroreflective sheeting 100 has multitude of cube-corner elements 120,shown as projecting from back side 138 of body layer 121, while bodylayer 121 is on front surface 125 of retroreflective sheeting 100. Asshown in FIG. 1B, light ray 160 enters cube-corner sheeting 100 throughfront surface 125, then passes through body layer 121 and strikes theplanar faces of cube-corner elements 120 and returns in the directionfrom which it came.

FIGS. 2 and 3 schematically illustrate exemplary embodiments of aportion of replicated composite cube-corner retroreflective sheeting 10.In the exemplary embodiment shown in FIG. 3, composite cube-cornerelements 12 surmount body layer 23, the lower or front surface 25 ofwhich is smooth or planar, and body layer 23 is contiguous withcomposite cube-corner elements 12, constituting what is referred to as a“land” portion. In some exemplary embodiments, lower or front surface 25may be roughened, such as described in PCT Application Nos.WO2010048416, published Apr. 29, 2010 (Smith et al.) and WO9630786,published Oct. 3, 1996 (Nilsen). The dimensions of the land portion ofthe sheeting relative to the individual cube-corner optical elementswill vary depending on the method chosen for manufacture and,ultimately, the end purpose of the sheeting. In some exemplaryembodiments, it is desirable to keep the land portion to a minimalthickness, as, for example, when flexibility of the sheeting isdesirable.

FIG. 4 depicts an alternative composition of a cube-cornerretroreflective sheeting 400 comprising light transmissive support layer421 and plurality of composite cube-corner elements 412. In contrast tothe cube-corner retroreflective sheeting of FIG. 3, cube-cornerretroreflective sheeting 400 has a minimal land portion (not shown). Inone exemplary embodiment, light transmissive support layer 421 is theoutermost layer on front side 425 of cube-corner retroreflectivesheeting 400. Light impinges on and passes through light transmissivesupport layer 421, strikes the planar faces of plurality of compositecube-corner elements 412 and returns in the direction from which itcame.

In the exemplary embodiment shown in FIG. 4, retroreflective sheeting400 comprises light transmissive support layer 421 and plurality ofcomposite cube-corner elements 412, each having apex 427 and base 438,and comprising first light transmissive polymeric layer 430 and secondlight transmissive polymeric layer 432. In the exemplary embodimentshown in FIG. 4, first light transmissive polymeric layer 430 comprisesapex 427, and second light transmissive polymeric layer 432 comprisesall of base 438. Typically, interface 436 is visually discernablebetween first light transmissive polymeric layer 430 and second lighttransmissive polymeric layer 432. Interface 436 is typically a curvedsurface, as shown.

Light transmissive support layer 421 may be secured directly to the baseof composite cube-corner elements 412, or it may be secured to thecomposite cube-corner elements by a land layer. In some embodiments, theland layer is kept to a minimal thickness and is made from a highelastic modulus material.

Light transmissive support layer 421 can comprise an overlay film, afabric, and/or a glass. In some exemplary embodiments, lighttransmissive support layer 421 is selected to be an overlay film havinga low elastic modulus (e.g., less than about 13×10⁸ pascals), and thecube-corner elements are selected to have a high elastic modulus (e.g.,greater than 16×10⁸ pascals), as described, for example, in U.S. Pat.No. 5,450,235 (Smith et al.). In some exemplary embodiments, theretroreflective sheeting 400 may have considerably greater flexibilitythan retroreflective sheeting 10 of FIG. 3.

In embodiments where the light transmissive support layer is selected tobe a fabric, the index of refraction of the fibers in the fabric isselected to substantially match the index of refraction of the secondlight transmissive in the composite cube-corners. Regarding matching ofthe index of refraction of fibers with polymers in optical elements,see, for example, U.S. Pat. No. 7,406,239 (Ouderkirk et al.), thedisclosure of which is incorporated herein by reference.

The light transmissive support layer typically comprises a low elasticmodulus polymer to impart easy bending, curling, flexing, conforming, orstretching to the resultant retroreflective composite. Generally, thelight transmissive support layer comprises a polymeric film having anelastic modulus of less than 13×10⁸ pascals, and a glass transitiontemperature less than about 50° C. The polymer preferably is such thatthe light transmissive support layer retains its physical integrityunder the conditions it is exposed to as the resultant compositeretroreflective sheeting is formed. The polymer desirably has a Vicatsoftening temperature that is greater than 50° C. The linear moldshrinkage of the polymer desirably is less than 1 percent, althoughcertain combinations of polymeric materials for the cube-corner elementsand the overlay film will tolerate a greater extent of shrinking of theoverlay film. Preferred polymeric materials are resistant to degradationby ultraviolet (“UV”) light radiation so that the retroreflectivesheeting can be used for long-term outdoor applications. The lighttransmissive support layer may be substantially transparent. Forinstance, films with a matte finish that become transparent when theresin composition is applied thereto, or that only become transparentduring the fabrication process (e.g., in response to the curingconditions used to form the array of cube-corner elements) are usefulherein.

The light transmissive support layer may be either a single layer ormulti-layer component as desired. If multilayer, the layer to which thearray of cube-corner elements is secured should have the propertiesdescribed herein as useful in that regard with other layers not incontact with the array of cube-corner elements having selectedcharacteristics as necessary to impart desired characteristics to theresultant composite retroreflective sheeting. Either surface of thelight transmissive support layer may contain printed or formed (e.g.,stamped or embossed) symbols and/or indicia, such as generally describedin U.S. Pat. No. 5,763,049 (Frey et al).

Exemplary polymers that may be employed in the light transmissivesupport layer used herein include fluorinated polymers, ionomericethylene copolymers, low density polyethylenes, plasticized vinyl halidepolymers, and polyethylene copolymers.

Exemplary fluorinated polymers include poly(chlorotrifluoroethylene)(e.g., such as that available from 3M Company, St. Paul, Minn., underthe trade designation “KEL-F800”),poly(tetrafluoroethylene-co-hexafluoropropylene) (e.g., such as thatavailable from Norton Performance, Brampton, Mass., under the tradedesignation “EXAC FEP”),poly(tetrafluoroethylene-co-perfluoro(alkyl)vinylether) (e.g., such asthat available from Norton Performance under the trade designation “EXACPEA”), and poly(vinylidene fluoride-co-hexafluoropropylene) (e.g., suchas that available from Pennwalt Corporation, Philadelphia, Pa., underthe trade designation “KYNAR FLEX-2800”).

Exemplary ionomeric ethylene copolymers includepoly(ethylene-co-methacrylic acid) with sodium or zinc ions (e.g., suchas those available from E.I. duPont Nemours, Wilmington, Del., under thetrade designations “SURLYN-8920” and “SURLYN-9910”).

Exemplary low density polyethylenes include low density polyethylene,linear low density polyethylene, and very low density polyethylene.

Exemplary plasticized vinyl halide polymers include plasticizedpoly(vinychloride).

Exemplary polyethylene copolymers that include acid functional polymersinclude (e.g., poly(ethylene-co-acrylic acid) (EAA),poly(ethylene-co-methacrylic acid) (EMA), poly(ethylene-co-maleic acid),and poly(ethylene-co-fumaric acid)), acrylic functional polymers (e.g.,poly(ethylene-co-alkylacrylates) where the alkyl group is methyl, ethyl,propyl, butyl, et cetera, or CH₃(CH₂)_(n) where n is 0 to 12), andpoly(ethylene-co-vinylacetate).

In some embodiments, the light transmissive support layer can includealiphatic and aromatic polyurethanes derived from the following monomers(i)-(iii):

-   (i) diisocyanates such as dicyclohexylmethane-4,4′-diisocyanate,    isophorone diisocyanate, 1,6-hexamethylene diisocyanate, cyclohexyl    diisocyanate, diphenylmethane diisocyanate, and combinations of    these diisocyanates;-   (ii) polydiols such as polypentyleneadipate glycol,    polytetramethylene ether gylcol, polycaprolactonediol,    poly-1,2-butylene oxide glycol, and combinations of these polydiols;    and-   (iii) chain extenders such as butanediol and hexanediol. Exemplary    urethane polymers include those available from Morton International    Inc., Seabrook, N.H., under the trade designations “PN-04” and    “3429”, and the urethane polymer available from B. F. Goodrich    Company, Cleveland, Ohio, under the trade designation “X-4107”.

The exemplary polymers that may be employed in the light transmissivesupport layer used herein may also be used in combination with eachother. Preferred polymers for the light transmissive support layerinclude: the ethylene copolymers that contain units that containcarboxyl groups or esters of carboxylic acids (e.g.,poly(ethylene-co-acrylic acid) (EAA), poly(ethylene-co-methacrylic acid)(EMA), poly(ethylene-co-vinylacetate)), ionomeric ethylene copolymers,plasticized poly(vinylchloride), and the aliphatic urethanes. Thesepolymers may be preferred, for example, for at least one of thefollowing reasons: suitable mechanical properties, good adhesions to thecomposite cube-corner layer, clarity, and environmental stability.

Referring again to the exemplary embodiment shown in both FIGS. 2 and 3,reference 12 generally designates one of the minute compositecube-corner elements of formations disposed in an array on one side ofsheeting 10. Each composite cube-corner element 12 has the shape of atrihedral prism with three exposed planar faces 22, substantiallyperpendicular to one another, with apex 27 of the prism typicallyvertically aligned with the center of the base. In some exemplaryembodiments, the apex 27 may be canted with respect to the center of thebase (see, e.g., U.S. Pat. No. 4,588,258 (Hoopman)) The angle betweenthe faces 22 is the same for each cube-corner element in the array, andwill be about 90°. Such angle can slightly deviate from 90° by design(i.e., the angle will be dependent upon the particular application ofthe sheeting), as described in U.S. Pat. No. 4,775,219 (Appeldorn).Cube-corner elements 12 typically have a height in the range of about 20to 500 micrometers, and more typically in the range of about 35 to 100micrometers.

A specular reflective coating (not shown) can be placed on the back side(opposite side from front side 425) of the cube-corner elements toenhance retroreflectivity. In some embodiments, a metallic coating canbe applied by known techniques such as vapor depositing or chemicallydepositing a metal such as aluminum, silver, or nickel. A primer layermay be applied to the back side of the cube-corner elements to promotethe adherence of the metallic coating. In addition to or in lieu of ametallic coating, a seal film can be applied to the back side of thecube-corner elements (see, e.g., U.S. Pat. No. 5,691,846 (Benson etal.), U.S. Pat. No. 5,784,197 (Frey et al.), U.S. Pat. No. 6,318,867(Bacon et al.), and U.S. Pat. No. 7,611,251 (Thakkar et al.)). The sealfilm maintains an air interface at the back side of the cubes to enhanceretroreflectivity. A backing or an adhesive layer can also be disposedbehind the cube-corner elements and/or seal film to secure cube-cornerretroreflective sheeting 400 to a substrate.

As is illustrated in FIG. 3, composite cube-corner elements 12 in sheet10 can be all of the same dimensions and aligned in a two-dimensionalarray or pattern of rows and columns, the bases being in the same plane,and adjacent elements being contiguous at the edges of their bases suchthat there are no margins or flat areas between adjacent elements, orspaced apart (not shown) as desired. If desired, different elements inthe array may have varying dimensions and orientations (e.g., the basesmay be tilted or otherwise in different planes with respect to eachother). In some embodiments (not shown) the protruding elements eachpossess more than one cube-corner apex. Various types of cube-cornerelements have been shown to be useful in providing retroreflectivearticles, including: truncated cubes (see, e.g., U.S. Pat. No. 4,588,258(Hoopman) and U.S. Pat. No. 5,138,488 (Szczech)), directly machinedcubes (see, e.g., U.S. Pat. No. 5,600,484 (Benson)), and fullcube-corners (see, e.g., U.S. Pat. No. 6,257,860 (Luttrell), and U.S.Pat. No. 7,556,386 (Smith)).

In the exemplary embodiment shown in FIG. 3, cube-corner elements 12 arecomposite cube-corners 35, each having apex 27 and base 38, andcomprising first light transmissive polymeric layer 30 and second lighttransmissive polymeric layer 32. In the exemplary embodiment shown inFIG. 3, first light transmissive polymeric layer 30 comprises apex 27,and second light transmissive polymeric layer 32 comprises all of base38. In some other exemplary embodiments (not shown), first lighttransmissive polymeric layer 30 may be a contiguous layer (within eachcube-corner element 12) that includes apex 27 and a first portion ofbase 38, and second light transmissive polymeric layer 32 includes asecond portion of base 38.

First light transmissive layer 30 and second light transmissive layer 32are selected to have a difference in refractive index. First lighttransmissive layer 30 has a first index of refraction, n₁, and secondlight transmissive layer 32 has a second index of refraction, n₂, andfirst index of refraction, n₁, and second index of refraction, n₂, havean absolute difference in refractive index of at least 0.0002 (i.e.,|n₁-n₂|≧0.0002). In some embodiments, the absolute difference inrefractive index is at least 0.001, at least 0.01, or even at least 0.1.In some embodiments, first index of refraction, n₁, is greater thansecond index of refraction, n₂, whereas in some other embodiments,second index of refraction, n₂, is greater than first index ofrefraction, n₁.

Typically, interface 36 is visually discernable between first lighttransmissive polymeric layer 30 and second light transmissive polymericlayer 32. Interface 36 is typically a curved surface, as shown.

Without being bound by theory, it is thought that retroreflectivearticles having composite cube-corner elements each having first andsecond light transmissive layers selected to have an absolute differenceof refractive index are useful for diverting retroreflected light tohigher observation angles than in monolithic cube-corner elements ofcomparable geometry and materials.

In composite cube-corner elements described herein, the percent byvolume (% by volume) of the first and second light transmissivepolymeric layers may be selected to obtain a variety of observationangle characteristics. In some embodiments, each of the comprisecube-corner elements may comprise up to 95% by volume of the secondlight transmissive polymeric layer, or up to 90% by volume, up to 75% byvolume, up to 60% by volume, up to 50% by volume, up to 25% by volume,or even up to 10% by volume, of the second light transmissive polymericlayer, the remainder of the volume of each of the comprise cube-cornerelements comprising the first light transmissive polymeric layer.

In the discussion of FIGS. 3 and 4, mention was made of interfaces 36and 436, respectively, as typically being visually discernable and alsotypically being curved. Without being bound by theory, it is thoughtthat during irradiation of a first radiation curable resin disposed incavities on a molding tool to form first light transmissive polymericlayer 31 (or 431), the radiation curable resin may undergoes some degreeof shrinkage due to bonds being formed in the first radiation curableresin, without pulling away from the walls of the cavity, and with theresulting formation of a curved surface in first light transmissivepolymeric layer 31 (or 431). Again, without being bound by theory, it isthought that the level of a first irradiation may be varied to inducevarying levels of bond forming reactions in the first radiation curableresin, which in turn results in varying degrees of curvature beingintroduced in the surface of first light transmissive polymeric layer 31(or 431).

The first light transmissive polymeric layer and the second lighttransmissive polymeric layer in the composite cube-corner elements aretypically formed from radiation curable resins capable of beingcrosslinked by a free radical polymerization mechanism by exposure toactinic radiation, (e.g., electron beam, ultraviolet light, or visiblelight).

Radiation-initiated cationically polymerizable resins also may be used.

In some embodiments, the first light transmissive polymeric layer andthe second light transmissive polymeric layer each include a lighttransmissive polymeric material, wherein the light transmissivepolymeric material in the first light transmissive polymeric layer is amore highly radiated form of the light transmissive material in thesecond light transmissive polymeric layer.

The radiation curable resin composition comprises one or morepolymerizable ethylenically unsaturated monomers, oligomers,prepolymers, or combination thereof. After curing, the ethylenicallyunsaturated components are reacted into a polymer. Preferredpolymerizable compositions are 100% solids and substantially free ofsolvent. Radiation curable resins suitable for forming the array ofcube-corner elements may be blends of photoinitiator and at least onecompound bearing an acrylate group. Preferably the resin blend containsa monofunctional, a difunctional, or a polyfunctional compound to ensureformation of a cross-linked polymeric network upon irradiation. In someembodiments of the method, the first radiation curable resin and thesecond radiation curable resin are each independently selected from thegroup consisting of monofunctional, difunctional, and polyfunctionalacrylates, and combinations thereof.

Exemplary radiation curable resins that are capable of being polymerizedby a free radical mechanism that can be used herein includeacrylic-based resins derived from epoxies, polyesters, polyethers, andurethanes, ethylenically unsaturated compounds, isocyanate derivativeshaving at least one pendant acrylate group, epoxy resins other thanacrylated epoxies, nitrogen-containing polymerizable resin composition(see, e.g., U.S. Pat. No. 7,862,187 (Thakkar et al.), the disclosure ofwhich is incorporated herein by reference), and mixtures andcombinations thereof. The term “acrylate” is used here to encompass bothacrylates and methacrylates. U.S. Pat. No. 4,576,850 (Martens) reportsexamples of crosslinked resins that may be used in cube-corner elementarrays of the present disclosure.

Ethylenically unsaturated resins include both monomeric and polymericcompounds that contain atoms of carbon, hydrogen and oxygen, andoptionally nitrogen, sulfur, and the halogens may be used herein. Oxygenor nitrogen atoms, or both, are generally present in ether, ester,urethane, amide, and urea groups. Ethylenically unsaturated compoundspreferably have a number average molecular weight of less than about4,000 and preferably are esters made from the reaction of compoundscontaining aliphatic monohydroxy groups, aliphatic polyhydroxy groups,and unsaturated carboxylic acids (e.g., acrylic acid, methacrylic acid,itaconic acid, crotonic acid, iso-crotonic acid, and maleic acid). Suchmaterials are typically readily available commercially and can bereadily cross linked.

Some exemplary compounds having an acrylic or methacrylic group that aresuitable for use in the radiation curable resins of the presentdisclosure include monofunctional compounds (e.g., ethylacrylate,n-butylacrylate, isobutylacrylate, 2-ethylhexylacrylate,n-hexylacrylate, n-octylacrylate, isooctyl acrylate, isobornyl acrylate,tetrahydrofurfuryl acrylate, 2-phenoxyethyl acrylate, andN,N-dimethylacrylamide), difunctional compounds (e.g., 1,4-butanedioldiacrylate, 1,6-hexanediol diacrylate, neopentylglycol diacrylate,ethylene glycol diacrylate, triethyleneglycol diacrylate, tetraethyleneglycol diacrylate, and diethylene glycol diacrylate), and polyfunctionalcompounds (e.g., trimethylolpropane triacrylate, glyceroltriacrylate,pentaerythritol triacrylate, pentaerythritol tetraacrylate, andtris(2-acryloyloxyethyl)isocyanurate).

Monofunctional compounds typically tend to provide faster penetration ofthe material of the overlay film, and difunctional and polyfunctionalcompounds typically tend to provide more crosslinked, stronger bonds atthe interface between the cube-corner elements and overlay film.

Some other exemplary ethylenically unsaturated compounds and resinsinclude styrene, divinylbenzene, vinyl toluene, N-vinyl formamide,N-vinyl pyrrolidone, N-vinyl caprolactam, monoallyl, polyallyl, andpolymethallyl esters (e.g., diallyl phthalate and diallyl adipate), andamides of carboxylic acids (e.g., N,N-diallyladipamide).

Cationically polymerizable materials including materials containingepoxy and vinyl ether functional groups may be used herein. Thesesystems are photoinitiated by onium salt initiators, such astriarylsulfonium, and diaryliodonium salts.

In one embodiment, the polymerizable resin comprises a combination of atleast one difunctional epoxy (meth)acrylate, at least one difunctional(meth)acrylate monomer, and at least one polyfunctional compound havingat least three (meth)acrylate groups.

The difunctional epoxy (meth)acrylate, as well as the difunctional(meth)acrylate monomer may be present in the polymerizable compositionin an amount of at least 5 wt. %, or at least 10 wt. %. Typically, theamount of such difunctional (meth)acrylate monomer does not exceed about40 wt. %. One exemplary epoxy diacrylate is available from CytecIndustries Inc., Smyrna, Ga., under the trade designation “EBECRYL3720”.

The polyfunctional compound is typically present in the polymerizablecomposition in an amount of at least 10 wt. %, at least 15 wt. %, atleast 20 wt. %, at least 25 wt. %, at least 30 wt. %, at least 35 wt. %,or even at least 40 wt. %. Typically, the amount of polyfunctionalcompound is not greater than about 70 wt. %.

Since methacrylate groups tend to be less reactive than acrylate groups,acrylate functionality is sometimes preferred.

Compositions curable by UV irradiation generally include at least onephotoinitiator. The photoinitiator can be used at a concentration in arange from 0.1 wt. % to 10 wt. %. More typically, the photoinitiator isused at a concentration in a range from 0.2 wt. % to 3 wt. %.

In general the photoinitiator is at least partially soluble (e.g., atthe processing temperature of the resin) and substantially colorlessafter being polymerized. The photoinitiator may be colored (e.g.,yellow), provided that the photoinitiator is rendered substantiallycolorless after exposure to the UV light source.

Suitable photoinitiators include monoacylphosphine oxide andbisacylphosphine oxide. Available mono or bisacylphosphine oxidephotoinitiators include 2,4,6-trimethylbenzoydiphenylphosphine oxide,available from BASF Corporation, Clifton, N.J., under the tradedesignation “LUCIRIN TPO”, ethyl-2,4,6-trimethylbenzoylphenylphosphinate, also available from BASF Corporation, under the tradedesignation “LUCIRIN TPO-L”, andbis(2,4,6-trimethylbenzoyl)-phenylphosphine oxide available from CibaSpecialty Chemicals, Tarrytown, N.Y., under the trade designation“IRGACURE 819”. Other suitable photoinitiators include2-hydroxy-2-methyl-1-phenyl-propan-1-one, available from Ciba SpecialtyChemicals, under the trade designation “DAROCUR 1173”, as well as otherphotoinitiators available from Ciba Specialty Chemicals, under the tradedesignations “DAROCUR 4265”, “IRGACURE 651”, “IRGACURE 1800”, “IRGACURE369”, “IRGACURE 1700”, and “IRGACURE 907”.

Free radical scavengers or antioxidants may be used, typically, in arange from about 0.01 wt. % to 0.5 wt. %. Suitable antioxidants includehindered phenolic resins such as those available from Ciba SpecialtyChemicals, under the trade designations “IRGANOX 1010”, “IRGANOX 1076”,“IRGANOX 1035”, and “IRGAFOS 168”.

The cube-corner or body layer composition may optionally comprise one ormore reactive (e.g., ethylenically unsaturated) ingredients and/or oneor more non-reactive ingredients. Various additives such as solvent,chain transfer agents, colorants (e.g., dyes), antioxidants, lightstabilizers, UV absorbers, processing aids such as antiblocking agents,releasing agents, lubricants, and other additives may be added to thebody layer or cube-corner elements as described in U.S. Pat. No.5,450,235 (Smith et al.).

Colorants, UV absorbers, light stabilizers, free radical scavengers orantioxidants, processing aids such as antiblocking agents, releasingagents, lubricants, and other additives may be added to one or both ofthe composite cube-corner elements and light transmissive support layerif desired. The particular colorant selected depends on the desiredcolor; colorants typically are added in a range from 0.01 wt. % to 0.5wt. %. UV absorbers typically are added in a range from 0.5 wt. % to 2wt. %. Suitable UV absorbers include derivatives of benzotriazole (e.g.,those available under the trade designations “TINUVIN 327”, “TINUVIN328”, “TINUVIN 900”, “TINUVIN 1130”, and “TINUVIN-P” from Ciba SpecialtyChemicals), chemical derivatives of benzophenone (e.g., those availableunder the trade designations “UVINUL M40”, “UVINUL 408”, and “UVINULD-50” from BASF Corporation, and “SYNTASE 230”, “SYNTASE 800”, and“SYNTASE 1200” from Neville-Synthese Organics, Inc., Pittsburgh, Pa.),or chemical derivatives of diphenylacrylate (e.g., available under thetrade designation “UVINUL N35” and “UVINUL 539” from BASF Corporation).Light stabilizers that may be used include hindered amines, which aretypically used in a range from 0.5 wt. % to 2 wt. %. Examples ofhindered amine light stabilizers include those available under the tradedesignations “TINUVIN 144”, “TINUVIN 292”, “TINUVIN 622”, “TINUVIN 770”,and “CHIMASSORB 944” from the Ciba Specialty Chemicals. Free radicalscavengers or antioxidants may be used, typically, in a range from 0.01wt. % to 0.5 wt. %. Suitable antioxidants include hindered phenolicresins such as those available under the trade designations “IRGANOX1010”, “IRGANOX 1076”, “IRGANOX 1035”, “MD-1024”, and “IRGAFOS 168”,available from the Ciba Specialty Chemicals. Small amounts of otherprocessing aids, typically no more than 1 wt. % of the polymer resins,may be added, for example, to improve the resin's processibility. Usefulprocessing aids include fatty acid esters, fatty acid amides available,for example, from Glyco Inc., Norwalk, Conn., or metallic stearatesavailable, for example, from Henkel Corp., Hoboken, N.J., as well asfrom Hoechst Celanese Corporation, Somerville, N.J., under the tradedesignation “WAX E”.

If desired, the polymeric materials of the retroreflective sheeting mayalso contain substances such as flame retardants that may enhancedesirable properties of the resultant sheeting as well as articles towhich it is attached.

Cube-corner retroreflective sheeting can be produced as is known in theart, including first manufacturing a master molding tool that has astructured surface, such structured surface corresponding either to thedesired cube-corner element geometry in the finished sheeting or to anegative (inverted) copy thereof, depending upon whether the finishedsheeting is to have cube-corner pyramids or cube-corner cavities (orboth). The molding tool is then replicated using any suitable techniquesuch as conventional nickel electroforming to produce tooling forforming cube-corner retroreflective sheeting by processes such asembossing, extruding, or cast-and-curing. Known methods formanufacturing the master molding tool include pin-bundling techniques,direct machining techniques, and techniques that employ laminae such asthose described in U.S. Pat. No. 7,188,960 (Smith). Variousmicroreplication methods for making cube-corner sheeting are described,for example, in U.S. Pat. No. 3,689,346 (Rowland), U.S. Pat. No.3,811,983 (Rowland), U.S. Pat. No. 4,332,847 (Rowland), U.S. Pat. No.4,601,861 (Pricone et al.), U.S. Pat. No. 5,491,586 (Phillips), U.S.Pat. No. 5,642,222 (Phillips), U.S. Pat. No. 5,691,846 (Benson et al.),U.S. Pat. No. 6,200,399 (Thielman), U.S. Pat. No. 7,410,604 (Erickson),and U.S. Pat. No. 7,611,251 (Thakkar et al.). These microreplicationprocesses produce a retroreflective sheeting with prismatic structuresthat have been precisely and faithfully replicated from amicrostructured molding tool having a negative image of the desiredprismatic structure. In some embodiments, the elements have a shape inplan view selected from trapezoids, rectangles, parallelograms,pentagons, and hexagons.

If the angles between faces of a replicated cube-corner element cannotbe controlled and maintained (e.g., because of shrinkage effects,distortion upon removal from the molding tool, or distortion due tothermal or mechanical stresses), the efficiency of retroreflection willbe materially affected. Even a slight lack of control and maintenance ofthe cube-corner geometry can significantly affect the resultantretroreflective efficiency.

The radiation curable resin may be poured or pumped, for example,directly into a dispenser that feeds a slot die apparatus.

The method of manufacturing the retroreflective sheeting of the currentdescription includes at least two irradiations of radiation curableresin. For example, the first radiation curable resin is pre-cured uponexposure to a suitable radiant energy source (i.e., actinic radiation),forming pre-cured partial cube-corner structures. Typically, thepre-cured material will have a degree of shrinkage so that a change inthe refractive index with respect to uncured radiation curable resin canbe detected, or it may become gelled, partially solidified, ornon-flowable. The second radiation curable resin is contacted onto thepre-cured first radiation curable resin, and both resins are thensubjected to a second irradiation (i.e., actinic radiation) tosufficiently harden all of the irradiated radiation curable resin priorto its removal from the tool.

In some embodiments, actinic irradiation sources typically includeultraviolet (“UV”) and electron beam irradiation. Suitable sources of UVirradiation include UV light emitting diodes (UV LEDs). Combinations ofcooling and curing may also be employed.

FIGS. 5A-5F are diagrammatical representations of a cube-corner recessin a molding tool in the progressive deposition and subsequent partialcuring from irradiation which occur during the production of a compositecube-corner in a retroreflective article of the present disclosure. FIG.5A represents cavity 527 in a surface of microreplicated tool 525. InFIG. 5B, first radiation curable resin 530 partially fills cavity 527.Upon a first radiation, first radiation curable resin 530 is pre-cured,forming a partially cured partial cube-corner structure 531, whichincludes cube-corner apex 537, as shown in FIG. 5C. In FIG. 5D, secondradiation curable resin 532 (which may be the same as or different fromfirst radiation curable resin 530) is brought into contact withpartially cured partial cube-corner structure 531, filling cavity 527and in the exemplary embodiment shown in FIG. 5D, light transmissivesupport layer 521 is in contact with second radiation curable resin 532.In some embodiments of the method, the second radiation curable resin isbrought into contact with a light transmissive support layer prior tothe second irradiation, wherein the second irradiation passes throughthe light transmissive support layer.

In some embodiments of the method, the first radiation curable resinshrinks by at least 5 percent by volume when cured. In some embodimentsof the method, the first radiation curable resin shrinks in a range from5 percent by volume to 20 percent by volume when cured.

In some embodiments of the method, the first radiation curable resin andsecond radiation curable resin are the same as each other.

In an alternate embodiment (not shown), the cavity may be over-filledwith a second radiation curable resin to form a land layer, which may becured to form a body layer, as was presented in FIG. 3 (see body layer323). In yet another alternate embodiment (not shown), the cavity may beover-filled with a second radiation curable resin to form a land layerand also have a light transmissive support layer overlaying the landlayer. FIG. 5E shows composite cube-corner 535 after a secondirradiation, having cured cube-corner structures 531 and 534 andinterface 536. FIG. 5F shows retroreflective article 500 removed fromthe molding tool.

In an alternate embodiment (not show) the first radiation curable resinis selectively applied (e.g., coated in a desired pattern) to a portionof the cube-corner cavities. Exemplary ways for applying the firstradiation curable resin to a portion of the cube-corner cavities in themolding tool in a desired pattern include contact printing, non-contactprinting, pattern coating, and combinations thereof. Examples of contactprinting include printing surface makes direct contact with a tool:direct and offset flexographic, direct and offset gravure, direct andoffset lithographic, direct and offset screen printing. Examples ofnon-contact printing include ink-jet, spray, acoustic, electrostatic,and digital deposition. Examples of pattern coating include patterneddie (for large rectangles) and needle (for downstream lines). An exampleof a combination of printing techniques is ink-jetting on a transferroll instead of on a tool.

In some embodiments, the light transmissive support layer may include anadhesion promoting surface treatment in order to enhance bonding to thecomposite cube-corner elements. Various adhesion promoting surfacetreatments are known and include mechanical roughening, chemicaltreatment, (e.g., air or inert gas (e.g., nitrogen)) corona treatment(e.g., such as described in U.S. Pat. No. 7,442,442 (Strobel et al.)),plasma treatment, flame treatment, and actinic radiation.

The process of forming retroreflective article 500 as show in FIGS.5A-5F may be carried out in either a batch mode or a continuous mode. Ina batch mode process, the molding tool may be a micro-structured tool,(e.g., a micro-structured film or a metal plate having a microstructurein a surface thereof). In a continuous mode process, the molding toolmay be, for example, a roll or a continuous belt having a microstructurein a surface thereof.

The molding tool of the present disclosure has a molding surface havinga plurality of cavities opening thereon which have the shape and sizesuitable for forming desired cube-corner elements. The opening at thetop surface of a cavity corresponds to the base of a resultantcube-corner element. The cavities, and thus resultant cube-cornerelements, may be three sided pyramids having one cube-corner each (see,e.g., U.S. Pat. No. 4,588,258 (Hoopman)), may have a rectangular basewith two rectangular sides and two triangular sides such that eachelement has two cube-corners each (see, e.g., U.S. Pat. No. 4,938,563(Nelson et al.)), or may be of other desired shape, having at least onecube-corner each (see, e.g., U.S. Pat. No. 4,895,428 (Nelson et al.)).It will be understood by those skilled in the art that any cube-cornerelement may be used in accordance with the present disclosure. Althoughthe present disclosure is described with particular reference tocomposite cube-corner elements, it will be understood that the articlesdescribed herein may include additional composite microstructuredreplicated elements that are secured to an overlay film in the manner ofthe composite cube-corner element discussed herein.

The molding tool should be such that the cavities will not deformundesirably during fabrication of the composite article, and such thatthe array of cube-corner elements can be separated therefrom aftercuring. Exemplary substrates known to be useful for forming molds forreplication of cube-corner elements include materials that can bedirectly machined. Such materials preferably machine cleanly withoutburr formation, exhibit low ductility and low graininess, and maintaindimensional accuracy after groove formation. A variety of machinableplastics, including both thermoset and thermoplastic materials (e.g.,acrylics), and machinable metals, including nonferrous metals (e.g.,aluminum, brass, copper, and nickel) are known. In many instances, itmay be desired to use a first or later generation replicate of amachined or shaped surface as the molding tool (i.e., the member onwhich the cube-corner sheeting of the invention is formed). Dependingupon the molding tool used and the nature of the resin composition, thecured array may separate from the molding tool readily or a partinglayer may be necessary to achieve desired separation characteristics.Exemplary parting layer materials include an induced surface oxidationlayer, an intermediate thin metallic coating, chemical silvering, andcombinations of different materials or coatings. If desired, suitableagents may be incorporated into the resin composition to achieve desiredseparation characteristics.

The molding tool can be made, for example, from polymeric, metallic,composite, or ceramic materials. In some embodiments, curing of theresin will be performed by applying radiation through the molding tool.In such instances, the molding tool should be sufficiently transparentto permit irradiation of the resin therethrough. Exemplary materialsfrom which molding tools for such embodiments can be made to includepolyolefins and polycarbonates. Metal molding tools are typicallypreferred, however, as they can be formed in desired shapes and provideexcellent optical surfaces to maximize retroreflective performance of agiven cube-corner element configuration.

FIG. 6 shows an exemplary embodiment of apparatus 600 having roll 625onto which is coated first radiation curable resin 630 from die 650,optionally using first roll 624 to aid in delivery of first radiationcurable resin 630, partially filling cavities 627 in the surface of roll625. As roll 625 rotates, first radiation curable resin 630 passes firstirradiation source 640, forming partially cured partial cube-cornerstructures 631, each of which is a first light transmissive polymericlayer of a composite cube-corner that is being formed. Second radiationcurable resin 632 (which may be the same as or different from firstradiation curable resin 630) is coated from die 652 onto lighttransmissive support layer 621 coming from supply roll 622, along withoptional light transmissive carrier film 628. Second radiation curableresin 632 is pressed into contact with partially cured partialcube-corner structures 631 with roll 623, and composite 633 (thatincludes light transmissive support layer 621, second radiation curableresin 632, and partially cured partial cube-corner structures 631)passes second irradiation source 641, forming composite cube-corners 635having first light transmissive polymeric layer 631 and second lighttransmissive polymeric layer 634 adhered to light transmissive supportlayer 621. The composite cube-corners on support layer are de-moldedfrom roll 625, and then pass post-cure irradiation source 642,completing formation of retroreflective article 610 having compositecube-corner elements 635, which for convenience is wound onto a take-uproll.

In an alternate method of making a retroreflective article (not show),an apparatus having a molding tool onto which is selectively applied afirst radiation curable resin is provided. In one embodiment, aflexographic printer is used to apply the first radiation curable resinin a desired pattern, partially filling at least a portion of thecube-corner cavities, forming partially filled partial cube-cornercavities and unfilled cube-corner cavities (i.e., cube-corner cavitiesthat are not filled by the first radiation curable resin). As themolding tool rotates, the partially filled cube-corner cavities and theunfilled cube-corner cavities are contacted by a second radiationcurable resin, different from the first radiation curable resin, whichfills the partially filled partial cube-corner cavities and completelyfills the unfilled cube-corner cavities, forming a composite. In someembodiments, the second radiation curable resin was coated from die ontoa light transmissive support layer. In other embodiments, the secondradiation curable resin was coated onto a carrier layer and the secondradiation curable resin over-filled the cube-corner cavities, forming aland layer. The land layer may be cured to form a body layer contiguouswith the base of the cube-corner elements, as was presented in FIG. 3(see body layer 323). The carrier layer could later be removed from thefinished article. In yet another alternate embodiment, the cavities maybe over-filled with a second radiation curable resin to form a landlayer and also have a light transmissive support layer overlaying theland layer. The composite passes a first irradiation source, formingcomposite cube-corners having first light transmissive polymeric layerand a second light transmissive polymeric layer, and monolithiccube-corners having a second light transmissive polymeric layer. In someembodiments the second light transmissive polymeric layer is adjacent alight transmissive support layer. In other embodiments the second lighttransmissive polymeric layer also formed a land layer. The first lighttransmissive polymeric layer and the second light transmissive polymericlayer of the composite cube-corners have, respectively, first index ofrefraction and second index of refraction, wherein the first and secondindices of refraction have an absolute difference of at least 0.0002.The composite and the monolithic cube-corners, having a land layerand/or a light transmissive support layer, are de-molded from themolding tool, and then pass a post-cure irradiation source, completingformation of a retroreflective article having composite and monolithiccube-corner elements.

In an embodiment of an exemplary retroreflective article having an arrayof composite cube corner elements in combination with monolithiccube-corner elements, and as shown in FIG. 9, retroreflective sheeting900 comprises light transmissive support layer 921 and plurality ofcomposite cube-corner elements 912, each having apex 927 and base 938,and comprising first light transmissive polymeric layer 930 and secondlight transmissive polymeric layer 932. In the exemplary embodimentshown in FIG. 9, first light transmissive polymeric layer 930 comprisesapex 927, and second light transmissive polymeric layer 932 comprisesall of base 938. Typically, interface 936 is visually discernablebetween first light transmissive polymeric layer 930 and second lighttransmissive polymeric layer 932. Interface 936 is typically a curvedsurface, as shown. Retroreflective sheeting 900 also comprisesmonolithic cube-corner elements 916, comprising second lighttransmissive polymer 932 in both the apex and the base of monolithiccube corner elements 916.

Light transmissive support layer 921 may be secured to base 938 ofcomposite cube-corner elements 912 and to the base of monolithiccube-corner elements 916, or it may be secured to the composite andmonolithic cube-corner elements by a land layer (not shown). In someembodiments, the land layer is kept to a minimal thickness and is madefrom a high elastic modulus material.

In an alternate method of making a retroreflective article (not show),an apparatus having a molding tool onto which is selectively applied afirst radiation curable resin is provided. In one embodiment, aflexographic printer is used to apply the first radiation curable resinin a desired pattern, partially filling at least a portion of thecube-corner cavities, forming partially filled partial cube-cornercavities and unfilled cube-corner cavities (i.e., cube-corner cavitiesthat were not filled by the first radiation curable resin). As themolding tool rotates, cube-corner cavities pass a first irradiationsource, and the first radiation curable resin is pre-cured formingpre-cured partial cube-corner structures. A second radiation curableresin (which may be the same as or different from first radiationcurable resin) is then pressed into contact with the pre-cured partialcube-corner structures and the unfilled cube-corner cavities with aroll, forming a composite. In some embodiments, the second radiationcurable resin was coated from die onto a light transmissive supportlayer. In other embodiments, the second radiation curable resin wascoated onto a carrier layer and the second radiation curable resinover-filled the cube-corner cavities, forming a land layer. The carrierlayer could later be removed from the finished article. In yet anotheralternate embodiment, the cavities may be over-filled with a secondradiation curable resin to form a land layer and also have a lighttransmissive support layer overlaying the land layer. The compositepasses a second irradiation source, forming composite cube-cornershaving first light transmissive polymeric layer and a second lighttransmissive polymeric layer, and monolithic cube-corners having asecond light transmissive polymeric layer. In some embodiments thesecond light transmissive polymeric layer is adjacent a lighttransmissive support layer. In other embodiments the second lighttransmissive polymeric layer also formed a land layer. The composite andmonolithic cube-corners, having a land layer and/or a light transmissivesupport layer, are de-molded from the molding tool, and then pass apost-cure irradiation source, completing formation of a retroreflectivearticle having composite and monolithic cube-corner elements.

In some embodiments, the retroreflective article of the presentdisclosure comprises composite cube-corner elements and monolithiccube-corner elements. In some embodiments, the composite cube-cornerelements create an optically variable mark. Methods for providing anoptically variable mark in retroreflective articles are also describedin U.S. Patent Application No. 61/491,602 (Attorney Docket No.67604US002), entitled “CUBE CORNER SHEETING HAVING OPTICALLY VARIABLEMARKING”, filed on the same date as the instant application, thedisclosure of which is incorporated herein by reference.

Methods of the present disclosure can be used to make a variety ofuseful retroreflective articles (e.g., traffic signs, barricades,license plates, pavement markers and marking tape, as well asconspicuity marking for vehicles and clothing).

Embodiments

-   Item 1. A retroreflective article comprising:

a light transmissive support layer having generally opposed first andsecond major surfaces; and

an array of composite cube-corner elements secured to the first majorsurface of the light transmissive support layer, wherein each compositecube-corner element comprises an apex and a base opposite the apex, andwherein each composite cube-corner element comprises a first lighttransmissive polymeric layer, a second light transmissive polymericlayer, and an interface therebetween;

wherein the first light transmissive polymeric layer comprises the apex,and the second light transmissive polymeric layer comprises at least aportion of the base,

-   wherein the first light transmissive polymeric layer has a first    index of refraction,-   wherein the second light transmissive polymeric layer has a second    index of refraction, and-   wherein the first and second indices of refraction have an absolute    difference of at least 0.0002.-   Item 2. The retroreflective article of item 1, wherein the first    index of refraction is greater than the second index of refraction.-   Item 3. The retroreflective article of item 1, wherein the first    index of refraction is less than the second index of refraction.-   Item 4. The retroreflective article of item 1, wherein each of the    composite cube-corner elements comprises up to 95% by volume of the    second light transmissive polymeric layer.-   Item 5. The retroreflective article of any of items 1 to 4, wherein    the first index of refraction and the second index of refraction    have an absolute difference of at least 0.001.-   Item 6. The retroreflective article of any of items 1 to 4, wherein    the first index of refraction and the second index of refraction    have an absolute difference of at least 0.01.-   Item 7. The retroreflective article of any of items 1 to 4, wherein    the first index of refraction and the second index of refraction    have an absolute difference of at least 0.1.-   Item 8. The retroreflective article of any of items 1 to 4, wherein    the first light transmissive polymeric layer and the second light    transmissive polymeric layer each comprise a light transmissive    polymeric material, and wherein the light transmissive polymeric    material in the first light transmissive polymeric layer is a more    highly irradiated form of the light transmissive material in the    second light transmissive polymeric layer.-   Item 9. A traffic sign that comprises a retroreflective article    according to any one of items 1 to 8.-   Item 10. A license plate that comprises a retroreflective article    according to any one of items 1 to 8.-   Item 11. A conspicuity film that comprises a retroreflective article    according to any one of items 1 to 8.-   Item 12. A method of making a retroreflective article, the method    comprising:

providing a molding tool having a microstructured surface including aplurality of cavities;

partially filling the plurality of cavities with a first radiationcurable resin, wherein the at least a portion of the plurality ofcavities comprises a cube-corner geometry;

exposing the first radiation curable resin to a first irradiation topre-cure the first radiation curable resin and provide pre-cured partialcube-corner structures;

contacting a second radiation curable resin onto the pre-cured partialcube-corner structures;

exposing the pre-cured partial cube-corner structures and the secondradiation curable resin to a second irradiation to provide compositecube-corners on the surface of the molding tool; and

separating the composite cube-corners from the surface of the moldingtool.

-   Item 13. The method of item 12, further comprising bringing the    second radiation curable resin into contact with a light    transmissive support layer prior to the second irradiation, wherein    the second irradiation passes through the light transmissive support    layer.-   Item 14. The method of item 12, wherein the light transmissive    support layer comprises a material selected from the group    consisting of film, fabric, and glass.-   Item 15. The method of any one of items 12 to 14, wherein the    molding tool is a micro-structured tool selected from the group    consisting of a roll, a continuous belt, a film, and a metal plate.-   Item 16. The method of any one of items 12 to 15, wherein the first    and second irradiations each independently include actinic    radiation.-   Item 17. The method of any one of items 12 to 16, wherein the first    radiation curable resin shrinks by at least 5 percent by volume when    cured.-   Item 18. The method of any one of items 12 to 17, wherein the first    radiation curable resin shrinks in a range from 5 percent by volume    to 20 percent by volume when cured.-   Item 19. The method of any one of items 12 to 18, wherein the second    radiation curable resin overfills the plurality of cavities and    forms a land layer.-   Item 20. The method of any one of items 12 to 19, wherein the    molding tool is light-transmissive.-   Item 21. The method of any one of items 12 to 20, wherein the first    radiation curable resin and the second radiation curable resin are    each independently selected from the group consisting of    monofunctional, difunctional, and polyfunctional acrylates, and    combinations thereof.-   Item 22. The method of any one of items 12 to 21, wherein the first    radiation curable resin and second radiation curable resin are the    same as each other.-   Item 23. A retroreflective article comprising:

a body layer having generally opposed first and second major surfaces;and

an array of composite cube-corner elements on the first major surface ofthe body layer, wherein each composite cube-corner element comprises anapex and a base opposite the apex, wherein each composite cube-cornerelement comprises a first light transmissive polymeric layer, a secondlight transmissive polymeric layer, and an interface therebetween,wherein the first light transmissive polymeric layer comprises the apex,wherein the second light transmissive polymeric layer is contiguous withthe body layer, wherein the first light transmissive polymeric layer hasa first index of refraction,

-   wherein the second light transmissive polymeric layer has a second    index of refraction, and-   wherein the first and second indices of refraction have an absolute    difference of at least 0.0002.-   Item 24. The retroreflective article of item 23, wherein the first    index of refraction is greater than the second index of refraction.-   Item 25. The retroreflective article of item 23, wherein the first    index of refraction is less than the second index of refraction.-   Item 26. The retroreflective article of item 23, wherein each of the    composite cube-corner elements comprises up to 95% by volume of the    second light transmissive polymeric layer.-   Item 27. The retroreflective article of any of items 23 to 26,    wherein the first index of refraction and the second index of    refraction have an absolute difference of at least 0.001.-   Item 28. The retroreflective article of any of items 23 to 26,    wherein the first index of refraction and the second index of    refraction have an absolute difference of at least 0.01.-   Item 29. The retroreflective article of any of items 23 to 26,    wherein the first index of refraction and the second index of    refraction have an absolute difference of at least 0.1.-   Item 30. The retroreflective article of any of items 23 to 26,    wherein the first light transmissive polymeric layer and the second    light transmissive polymeric layer each comprise a light    transmissive polymeric material, and wherein the light transmissive    polymeric material in the first light transmissive polymeric layer    is a more highly irradiated form of the light transmissive material    in the second light transmissive polymeric layer.-   Item 31. A traffic sign that comprises a retroreflective article    according to any one of items 23 to 30.-   Item 32. A license plate that comprises a retroreflective article    according to any one of items 23 to 30.-   Item 33. A conspicuity film that comprises a retroreflective article    according to any one of items 23 to 30.-   Item 34. A method of making a retroreflective article, the method    comprising:

providing a molding tool having a microstructured surface including aplurality of cube-corner cavities;

applying a first radiation curable resin to a portion of the cube-cornercavities in a desired pattern, partially filling a portion of thecube-corner cavities and forming partially filled cube corner cavitiesand unfilled cube-corner cavities;

contacting the partially filled cube corner cavities and unfilledcube-corner cavities with a second radiation curable resin, wherein thesecond radiation curable resin is different from the first radiationcurable resin, forming a composite;

exposing the composite to an irradiation source to provide compositecube-corners and monolithic cube-corners on the surface of the moldingtool; and

separating the composite cube-corners and monolithic cube-corners fromthe surface of the molding tool.

-   Item 35. A method of making a retroreflective article, the method    comprising:

providing a molding tool having a microstructured surface including aplurality of cube-corner cavities;

applying a first radiation curable resin to a portion of the cube-cornercavities in a desired pattern, partially filling a portion of thecavities and forming partially filled cube corner cavities and unfilledcube-corner cavities;

exposing the composite to a first irradiation to provide pre-curedpartial cube-corner structures;

contacting the pre-cured partial cube-corner structures and unfilledcube-corner cavities with a second radiation curable resin, forming acomposite;

exposing the composite to a second irradiation to provide compositecube-corners and monolithic cube-corners on the surface of the moldingtool; and

separating the composite cube-corners and monolithic cube-corners fromthe surface of the molding tool.

-   Item 36. A retroreflective article comprising:

a light transmissive support layer having generally opposed first andsecond major surfaces; and

an array of composite cube-corner elements and monolithic cube-cornerelements secured to the first major surface of the light transmissivesupport layer, each cube-corner element comprising an apex and a baseopposite the apex;

wherein each composite cube-corner element comprises a first lighttransmissive polymeric layer comprising the apex, the first lighttransmissive polymeric layer having a first index of refraction, and asecond light transmissive polymeric comprising at least a portion of thebase, the second light transmissive polymeric layer having a secondindex of refraction, and wherein the first and second indices ofrefraction have an absolute difference of at least 0.0002.

-   Item 37. The retroreflective article of item 36, wherein the    monolithic cube-corner elements comprise the second light    transmissive polymeric layer.-   Item 38. The retroreflective article of any of items 36 or 37,    wherein the second light transmissive polymeric layer is different    from the first light transmissive polymeric layer.-   Item 39. The retroreflective article any of items 36 to 38, wherein    the composite cube-corner elements form an optically variable mark.

Advantages and embodiments of this invention are further illustrated bythe following examples, but the particular materials and amounts thereofrecited in these examples, as well as other conditions and details,should not be construed to unduly limit this invention. All parts andpercentages are by weight unless otherwise indicated.

EXAMPLES

Test Methods

Measuring Brightness at Various Observation Angles

The coefficient of retroreflection, R_(A) was measured following theprocedure outlined in ASTM E-810-03 “Test Method for Coefficient ofRetroreflection of Retroreflective Sheeting Utilizing the CoplanarGeometry” (approved February, 2008), incorporated herein by reference.R_(A) was measured at discrete observation angles and averaged over theannular region between two adjacent measured observation angles.

Measuring Fractional Retroreflectance and Fractional RetroreflectanceSlope

Incremental % R_(T) for a given observation angle was determined bymultiplying the averaged R_(A) by the area of the annular region dividedby the cosine of the entrance angle. Fractional retroreflectance % R_(T)was calculated as the sum of incremental % R_(T) for observation anglesbetween 0 and the observation angle of interest (α[max]). Fractionalretroreflectance slope for a given observation angle was the incremental% R_(T) divided by the difference between the adjacent observationangles.

Materials

BAED bisphenol-A epoxy diacrylate obtained from Cytec Industries Inc.,Smyrna, GA, under the trade designation “EBECRYL 3720”. DMAEADimethylaminoethyl acrylate, obtained from Cytec Industries Inc. EAAEthylene acid acrylate, obtained from Dow Company, Midland, MI, underthe trade designation “PRIMACOR 3340”. HDDA 1,6-hexanediol diacrylate,obtained from Cytec Industries Inc. HMP/TPO Blend of2-hydroxy-2-methyl-1-phenyl propan-1-one and2,4,6-trimethyl-benzoyldiphenylphosphine oxide (TPO), obtained from BASFCorporation, Florham Park, NJ, under the trade designation “DAROCUR4265” TMPTA Trimethylolpropane triacrylate, obtained from CytecIndustries Inc. TPO (2,4,6 trimethylbenzoyl) diphenylphosphine oxide, aphotoinitiator, obtained from Sigma-Aldrich, St. Louis, MO.Preparation of Composition 1

A first radiation-curable resin (Composition 1) was prepared bycombining 25 wt. % BAED, 12 wt. % DMAEA, 38 wt. % TMPTA, 25 wt. % HDDA,and 0.5 pph (parts per hundred) TPO.

Preparation of Composition 2

A second radiation curable resin (Composition 2) was prepared by mixing90 wt. % of Composition 1 with 10 wt. % of BAED.

Preparation of Composition 3

A third radiation curable resin (Composition 3) was prepared by mixing50 wt. % of TMPTA, 25 wt. % of HDDA, 25 wt. % of BAED, and 1 pph ofHMP/TPO.

Illustrative Examples 1-3

Illustrative Examples 1-3 were prepared by coating a film of a givencomposition at a thickness of about 3 mil (75 micrometers) onto a firstpolyester terephthalate (PET) film. Pre-cured films were prepared byirradiating the coated films for about 15 seconds using an array oflight emitting diode (LED) lamps (obtained as Model “LN 120-395B-120”from Clearstone Technologies, Minneapolis, Minn.), emitting in the 395nanometer wavelength range and having an energy output level at a 100%power setting of about 170 milliwatts per square centimeter (mW/cm²). Asecond 3 mil (75 micrometers) thick layer of the same composition wasthen coated onto the partially cured coated film. A second PET film wasplaced over the second layer to form a composite, and the composite wasirradiated through the second PET film with two Fusion “D” UV lamps(obtained from Fusion Systems, Rockville, Md.) set at 600 Watts persquare inch (W/in²) for about 15 seconds. Dichroic filters were used infront of the UV lamps. Both the first and the second PET films were thenremoved from the composite.

Refractive indices of Illustrative Examples 1-3 were measured on eachmajor side of the composite using a refractometer (obtained as Model2010/M from Metricon Corporation, Pennington, N.J.) equipped with alaser diode control unit set at 404 nm (obtained from Power Technology,Little Rock, Ark.). Compositions and refractive indices for each majorside of the composites of Illustrative Examples 1-3 are reported inTable 1, below.

TABLE 1 Illustrative Radiation Refractive Index Example Curable ResinLED-cured side UV-cured side 1 Composition 1 1.5239 1.5228 2 Composition2 1.5266 1.5242 3 Composition 3 1.5246 1.5209

Comparative Example A

Comparative Example A was a white retroreflective sheeting was obtainedfrom 3M Company, St. Paul, Minn., under the trade designation “3MENGINEER GRADE REFLECTIVE SHEETING 3290”.

Example 4

The following description for the preparation of Example 4 refers toapparatus 600 as generally shown in FIG. 6. An overlay film 621 was madeby extruding an EAA film at a thickness of 0.01 cm (4 mil) onto a coronatreated polyethylene terephthalate (PET) carrier film 628. Pellets ofEAA were fed into a 1.9 cm (0.75 in.) single screw extruder (obtainedfrom C.W. Brabender Instruments Inc., South Hackensack, N.J.) withtemperatures set at 140° C. (284° F.) for zone 1 and ramped up to 175°C. (347° F.) at the extruder exit and die, resulting in a melttemperature of about 175° C. (347° F.). As the molten resin exited theextruder, it passed through a conventional horizontal film die (obtainedfrom Extrusion Dies Industries LLC, Chippewa Falls, Wis., under thetrade designation “ULTRAFLEX-40”) and was cast onto the PET carrier film628. The PET carrier film 628 was traveling at about 36 meters/min (120ft/min) The resulting molten overlay film 621 on the PET carrier film628 was run between a rubber roll/chilled steel roll nip to solidify themolten resin into a layer. The EAA surface was corona treated at anenergy level of about 1.0 J/cm².

A first portion of Composition 1 (630) was extruded and passed through afirst die 650 which was brought into close proximity to a first rubberroll 624. The rubber roll 624 ran in a clockwise motion and nippedagainst a molding tool 625 heated to 180° F. (82° C.) containing aplurality of cube-corner cavities 627. The molding tool 625 was mountedon a mandrel rotating in a counterclockwise motion at about 75 fpm (22.8m/min) Composition 1 (630) partially filled the cube-corner cavities toabout 60% in volume, providing partially filled partial cube-cornerstructures 631. The partially filled partial cube-corner structures 631were then pre-cured with the array of light emitting diodes (LEDs) 640placed about 1.2 inch (3 cm) from the molding tool 625. The array ofLEDs 640 was controlled by a controller (obtained as Model CF2000 fromClearstone Technologies) (not shown), at a controller power setting of10%. The overlay film 621 was drawn along from a supply roll 622 withthe EAA side facing upward. A second portion of Composition 1 (632) wassimultaneously cast through a second die 652 onto a second rubber roll624. The second rubber roll 624 contacted the EAA side of the overlayfilm 621, transferring the second portion of the Composition 1 (632)onto the overlay film. The coated overlay film was brought in contactwith the molding tool 625 containing the partially filled, pre-curedcube-corner structures 631 via a third silicone-coated rubber roll 623.The resin coated on the overlay film completely filled the unfilledportion of the cube-corner cavities, and the composite construction wascured through the overlay film 621 to form a retroreflective filmarticle 634, using two Fusion “D” lamps 641 (Fusion Systems) set at 600W/in., and also using dichroic filters (not shown) in front of the UVlamps. The retroreflective film 634 was separated from the molding tool625 and then was irradiated by a Fusion “D” UV lamp 642 operating at100% to provide a post-UV irradiation cure through the compositecube-corner structures 635. The retroreflective film 634 was then passedthrough an oven set at 127° C. (260° F.).

The resulting cube-corner structures 635 had three sets of intersectinggrooves with a pitch of 3.2 mils (81 micrometers). The intersectinggrooves formed a cube-corner base triangle with included angles of 61°and a cube-corner element height of 1.95 mil (50 micrometers). Theprimary groove spacing is defined as the groove spacing between thegrooves which form the two 61° base angles of the base triangle.

Example 5

Example 5 was prepared as described in Example 4, except that acontroller power setting of 25% was used.

Example 6

Example 6 was prepared as described in Example 4, except that acontroller power setting of 50% was used.

Example 7

Example 7 was prepared as described in Example 4, except that acontroller power setting of 90% was used.

Comparative Example B

Comparative Example B was prepared as described in Example 4, exceptthat a controller power setting of 0% was used (i.e., no pre-cure of thepartially filled partial cube-corner structures 631 was performed).

Retroreflectivity (R_(A)) values for Comparative Examples A-B andExamples 4-7 were measured at an observation angle of 0.2 degrees,entrance angle of −4 degrees, and orientation of 0 degrees. Results areshown in Table 2, below (the controller power setting for LEDS was notapplicable (N/A) for Comparative Example A, which was tested asobtained).

TABLE 2 Controller Power Setting for LEDs Examples (%) R_(A) (cd/lux ·m²) Comparative N/A 90 Example A Comparative  0 922.7 Example B Example4 10 897.1 Example 5 25 730.1 Example 6 50 653.2 Example 7 90 622.1

Example 8

Example 8 was prepared as described in Example 4, except that acontroller power setting of 25% was used.

Example 9

Example 9 was prepared as described in Example 4, except that acontroller power setting of 50% was used.

Example 10

Example 10 was prepared as described in Example 4, except that acontroller power setting of 75% was used.

Example 11

Example 11 was prepared as described in Example 4, except that acontroller power setting of 100% was used.

FIGS. 7 and 8, respectively, show % RT and % RT Slope values forComparative Examples A and B, and Examples 8-11.

Foreseeable modifications and alterations of this disclosure will beapparent to those skilled in the art without departing from the scopeand spirit of this invention. This invention should not be restricted tothe embodiments that are set forth in this application for illustrativepurposes.

What is claimed is:
 1. A retroreflective article comprising: a lighttransmissive support layer having generally opposed first and secondmajor surfaces; and an array of composite cube-corner elements securedto the first major surface of the light transmissive support layer,wherein each composite cube-corner element comprises an apex and a baseopposite the apex, and wherein each composite cube-corner elementcomprises a first light transmissive polymeric layer, a second lighttransmissive polymeric layer, and an interface therebetween; wherein thefirst light transmissive polymeric layer comprises the apex, and thesecond light transmissive polymeric layer comprises at least a portionof the base, wherein the first light transmissive polymeric layer has afirst index of refraction, wherein the second light transmissivepolymeric layer has a second index of refraction, wherein the first andsecond indices of refraction have an absolute difference of at least0.0002, wherein the first light transmissive polymeric layer and thesecond light transmissive polymeric layer in the composite cube-cornerelements are configured to divert retroreflected light to higherobservation angles than in monolithic cube-corner elements of comparablegeometry and materials, and wherein each of the composite cube-cornerelements comprises up to 95% by volume of the second light transmissivepolymeric layer.
 2. The retroreflective article of claim 1, wherein thefirst index of refraction and the second index of refraction have anabsolute difference of at least 0.001.
 3. The retroreflective article ofclaim 1, wherein the first light transmissive polymeric layer and thesecond light transmissive polymeric layer each comprise a lighttransmissive polymeric material, and wherein the light transmissivepolymeric material in the first light transmissive polymeric layer is amore highly irradiated form of the light transmissive material in thesecond light transmissive polymeric layer.
 4. A traffic sign thatcomprises a retroreflective article according to claim
 1. 5. A licenseplate that comprises a retroreflective article according to claim
 1. 6.A conspicuity film that comprises a retroreflective article according toclaim
 1. 7. The article of claim 1, wherein the composite cube-cornerelements form an optically variable mark.
 8. A retroreflective articlecomprising: a body layer having generally opposed first and second majorsurfaces; and an array of composite cube-corner elements on the firstmajor surface of the body layer, wherein each composite cube-cornerelement comprises an apex and a base opposite the apex, wherein eachcomposite cube-corner element comprises a first light transmissivepolymeric layer, a second light transmissive polymeric layer, and aninterface therebetween, wherein the first light transmissive polymericlayer comprises the apex, wherein the second light transmissivepolymeric layer is contiguous with the body layer, wherein the firstlight transmissive polymeric layer has a first index of refraction,wherein the second light transmissive polymeric layer has a second indexof refraction, wherein the first and second indices of refraction havean absolute difference of at least 0.0002, wherein the first lighttransmissive polymeric layer and the second light transmissive polymericlayer in the composite cube-corner elements are configured to divertretroreflected light to higher observation angles than in monolithiccube-corner elements of comparable geometry and materials, and whereineach of the composite cube-corner elements comprises up to 95% by volumeof the second light transmissive polymeric layer.
 9. The retroreflectivearticle of claim 8, wherein the first index of refraction and the secondindex of refraction have an absolute difference of at least 0.001. 10.The retroreflective article of claim 8, wherein the first lighttransmissive polymeric layer and the second light transmissive polymericlayer each comprise a light transmissive polymeric material, and whereinthe light transmissive polymeric material in the first lighttransmissive polymeric layer is a more highly irradiated form of thelight transmissive material in the second light transmissive polymericlayer.
 11. A traffic sign that comprises a retroreflective articleaccording to claim
 8. 12. A license plate that comprises aretroreflective article according to claim
 8. 13. A conspicuity filmthat comprises a retroreflective article according to claim
 8. 14. Thearticle of claim 8, wherein the composite cube-corner elements form anoptically variable mark.
 15. A method of making a retroreflectivearticle, the method comprising: providing a molding tool having amicrostructured surface including a plurality of cube-corner cavities;applying a first radiation curable resin to a portion of the cube-cornercavities in a desired pattern, partially filling a portion of thecavities and forming partially filled cube corner cavities and unfilledcube-corner cavities; exposing the first radiation curable resin to afirst irradiation to provide pre-cured partial cube-corner structures;contacting the pre-cured partial cube-corner structures and unfilledcube-corner cavities with a second radiation curable resin, forming acomposite; exposing the composite to a second irradiation to providecomposite cube-corners and monolithic cube-corners on the surface of themolding tool; and separating the composite cube-corners and monolithiccube-corners from the surface of the molding tool.
 16. The method ofclaim 15, further comprising bringing the second radiation curable resininto contact with a light transmissive support layer prior to the secondirradiation.